1. Field of the Invention
The present invention relates to a light-emitting element driving circuit and a device using the same, and more particularly to a light-emitting element driving circuit which can operate at a high speed and can drive a light-emitting element directly by a logic circuit or by adding a simple circuit to a conventional circuit configuration.
Recently, there is a considerable activity in the developments of light-emitting elements such as semiconductor laser diodes. Such light-emitting elements are widely used and applied to, for example, optical communication systems and optical disk drives. For example, an optical repeater employs a semiconductor laser diode, which converts an electric signal into an optical signal. A light-emitting element driving circuit is used to drive the light-emitting element such as a semiconductor laser diode. More particularly, the light-emitting element driving circuit supplies the light-emitting element with a pulse current (which turns ON and OFF the element) and a bias current (which defines the magnitude of the output light of the element).
2. Description of the Related Art
The applicant proposed an improved light-emitting element driving circuit in related U.S. patent application Ser. No. 810,710 filed Mar. 3, 1997, the disclosure of which is hereby incorporated by reference. The improved circuit proposed in the Application was made in order to solve the problems of light-emitting element driving circuits of a current drawing type, which is unsuitable for a recent situation in which only a positive power supply is used to supply electricity with circuits. The proposed circuit has a configuration of a current supplying type. This will now be described with reference to FIG. 1.
FIG. 1 shows a light-emitting element driving circuit proposed in the above Application. The circuit is made up of a resistor 12, a constant-current source 15, a signal input terminal 16 and a logic circuit 14. The logic circuit 14 includes a drive transistor 13, and is, for example, MC100E416 produced by Motorola. An input signal which controls the light-emitting element 11 is applied to a control terminal (gate) of the drive transistor 13 via the input terminal 16. The drive transistor 13 converts the voltage of the input signal into a current, which flows in the light-emitting element 11. The above current includes a pulse current Ip and a first bias current Ibt. The resistor 12 adjusts the pulse current Ip and the first bias current Ibt. The constant-current source 15 is provided on the anode side of the light-emitting element 11, and supplies a second bias current Ibc thereto.
The light output of the light-emitting element 11 depends on the total bias current Ic which corresponds to the sum of the first and second bias currents Ibt and Ibc (Ic=Ibc+Ibt). The light-emitting element 11 is turned ON and OFF on the basis of the pulse current Ip dependent on the input signal.
In the circuit shown in FIG. 1, the bias current Ib is mainly supplied from the constant-current source 15 and is also supplied from a pulse current supply part made up of the resistor 12 and the drive transistor 13. The bias current Ib supplied to the light-emitting element 11 is defined as follows: EQU Ib=Ibc+Ibt (1) EQU =Ibc+(Vout(L)-.phi..sub.LD)/R.sub.LD (2)
where Vout(L) denotes the low-level output of the logic circuit 14, .phi..sub.LD denotes a built-in voltage of the light-emitting element 11, and R.sub.LD denotes a resistance value of the resistor 12.
The light output Pout of the light-emitting element 11 is generally expressed as follows: EQU Pout=(Ip+Ib-Ith)*.delta. (3)
where Ith denotes the threshold current of the light-emitting element 11, and .delta. denotes the differential quantization efficiency.
There are two disadvantages to be solved.
If the power supply voltage VCC is varied, the low-level output Vout(L) will be varied and thus the light output will be varied. Hence, there is a very limited variation range of the power supply voltage VCC in which the suitable light output can be obtained. Thus, the light-emitting element driving circuit shown in FIG. 1 can be used in restricted applications. The above is the first disadvantage.
The above equation (2) does not have any term dependent on temperature. Hence, the bias current Ib is substantially constant irrespective of temperature variations. However, if a light-emitting element has the threshold voltage Ith or the differential quantization efficiency .delta. depending on temperature, the light output of the element will be varied due to temperature variations irrespective of whether the bias current Ib is constant. Hence, such a light-emitting element is applied to very restricted environments which do not require stability of the element with respect to temperature variations. The above is the second disadvantage.